The purpose of this proposal is to delineate the role of NF-(B subunits in the development of inflammatory bowel disease. NF-(B is a transcription factor family consisting of 5 subunits. Activation of NF-(B in response to Toll-like receptor (TLR) signaling plays a central role in driving the pathological inflammation that characterizes inflammatory bowel disease. However, it has been shown that in addition to pro-inflammatory effects, individual subunits can have unexpected anti-inflammatory effects as well. Mice lacking the p50/p105 subunit of NF-(B are sensitized to the development of colitis, due at least in part to a defect within hematopoietic cells of the innate immune system that leads to excessive production of IL-12 p40. In contrast, mice lacking the NF-(B subunit c-Rel, are resistant to colitis, and have reduced expression of IL-12 p40. Thus, NF-(B family members p50/p105 and c-Rel appear to have opposing functions in the regulation of lower bowel inflammation. The proposal contains 2 aims. Aim 1: To determine the basis for increased expression of IL-12 p40 in p50/p105-deficient macrophages. We have found that increased TLR- induced expression of IL-12 p40 in macrophages that lack p50/p105 is associated with augmented secretion of interferon-2 (IFN-2) and enhanced activation of STAT1. However, the relationship between augmented production of IFN-2 and increased expression of IL-12 p40 has not been define. One possibility is that increased expression of IL-12 p40 is secondary to augmented production of IFN-2 and enhanced activation of STAT1. An alternative possibility is that increased expression of IL- 12 p40 observed in p50-deficient macrophages is independent of IFN-2 secretion, based on the direct ability of p50 to inhibit expression of IL-12 p40. The purpose of this aim is to examine these possibilities and delineate the inhibitory function of p50. Aim 2: To characterize the role of c-Rel in regulating the mucosal inflammatory response. The NF-(B family member c-Rel plays an essential role in regulating induction of both IL-23 subunits, p40 and p19 (11, 12). Consistent with a role for c- Rel in regulating IL-23 expression in vivo, we have found that mice lacking c-Rel within the innate immune system are resistant to the development of T cell-mediated colitis, and exhibit reduced expression of both interferon-3 and IL-17 (8). This has lead us to hypothesize that the presence of c- Rel within the innate immune system is necessary for induction of Th1 and Th17 mediated bowel pathology. In this aim, we intend to evaluate this hypothesis by determining how the absence of c-Rel within the innate immune system interferes with the differentiation and activation of effector and regulatory T cell populations using an in vivo colitis model and in vitro culture system. These studies will advance our understanding of IBD and possibly suggest novel therapeutic approaches. PUBLIC HEALTH RELEVANCE: NF-(B is a central mediator of the pathology that characterizes inflammatory bowel disease. A detailed understanding of the function of the NF-(B family members may unveil novel therapeutic approaches.